Engine housings for various types of engines are well known. Liquid-cooled engines are usually provided with a radiator containing a heat conducting liquid, a housing of some type usually surrounding the radiator and/or engine and means for connecting the engine and radiator so that the liquid in the radiator is circulated through the engine to absorb engine heat. A fan is usually provided to assist the heat transfer at the radiator.
A housing for an air-cooled engine poses a special problem since the engine is not equipped with a radiator to help dissipate engine heat. In practice, many machines powered by an air-cooled engine have no engine housing at all, and engine heat is allowed to dissipate directly from the engine into the atmosphere. Some examples of these machines are commercial-type filament trimmers, debris blowers, and chain saws.
There are obvious disadvantages to operating an air-cooled engine without a housing of some type; the danger of burns from direct contact with the engine; the danger of starting a fire from contact of the engine with some combustible material; and possible damage to the engine itself, from contact with some other object. In an air-cooled engine, the cooling fins are relatively fragile, and breakage is fairly common where they are exposed. In addition, an exposed engine is not as aesthetically pleasing as a well designed housing.
Some manufacturers of small, gasoline engine powered implements such as trimmers or chain saws, have provided a shroud or partial housing over the engine. However, for dissipating engine heat, a muffler or cooling fins, or both, are normally exposed to the atmosphere, and consequently to the user. Some attempts have been made to provide a housing to fully enclose a small, air-cooled engine. However, until the teachings of the present invention, these attempts have not been totally successful because of the variety of problems which may be encountered over a range of operating conditions. For example, a tiller being operated in an ambient temperature of 90.degree. Fahrenheit may be subject to vapor lock because of recirculation of engine heat in the housing. A snow thrower powered by the same engine in the same housing, operating in an ambient temperature below freezing, may be subject to carburetor icing because of the extremely cold air entering the housing.